This is a non-randomized, open label, multi-site, single-dose, Phase 1/2 study in up to 18 subjects (including at least 3 adolescents between 12 and 17 years of age, inclusive) with beta-thalassemia major. The study will evaluate the safety and efficacy of autologous hematopoietic stem cell transplantation (HSCT) using LentiGlobin® BB305 Drug Product [autologous CD34+ hematopoietic stem cells transduced with LentiGlobin® BB305 Lentiviral Vector encoding the human β-A(T87Q)-globin gene].